The present invention relates to surge arresters and/or transient voltage surge suppressor devices, and in particular, to surge arresters or suppressors that employ Metal Oxide Varistors, Silicon Avalanche Diodes, Selenium Rectifiers, Silicon Controlled Rectifiers, Triacs and other semiconductor devices, singly or in combination. The term MOV will describe the application of Metal Oxide Varistors specifically but will also be employed as a generally representative term for the equivalent application of the above mentioned technologies.
The objective of a surge arrester is to provide a low impedance path for a high voltage transient at a defined voltage level. Some surge arresters, especially those for power conditioning circuits, employ a multiplicity of Metal Oxide Varistor modules to accomplish this task.
Metal Oxide Varistors are surge protection devices that are connected across power circuits and provide protection for equipment connected to such power circuits from temporary voltage peaks and transients.
Metal oxide Varistors have a standard operating mode for normal conditions where the voltage across the circuit is stable and a clamping mode for transient and overvoltage conditions. During clamping mode the MOV acts to reduce the maximum voltage across the circuit to a predefined level.
There are several problems with MOV based surge arresters. One problem is that MOV modules when subjected to an excessive voltage will overheat rendering the surge arrester inoperable. For example, when power sources are subject to long duration surge conditions, these lines may produce large voltages and associated currents large. The MOV when subject to these extreme conditions will overheat, rupture, and/or produce noxious gases and/or particles. Further, the ruptured MOV will permit a plasma arc to occur between the sections of the MOV thereby endangering the electronic device to which the surge arrester is attached.
Furthermore, over time and through use, MOV modules deteriorate thereby lowering the maximum voltage the modules can adequately handle. Specifically, when the clamping threshold of the MOV module deteriorates to the point where it descends into the peak of the power line sine-wave voltage the module will overheat and fail in the manner described above. This effect may also occur if the clamp threshold descends to at or slightly below the DC level.
Additionally, MOV's can fail in an essentially short circuit mode when subjected to excessive transient peak currents. For example, conventional 40 mm diameter MOV's are rated to handle 40 kA, 8/20 microsecond waveforms for 1 event. At peak currents in excess of 40 kA the MOV will fail as a "short circuit". If the MOV is not properly fused, excessively current flow through the MOV will cause it to rupture and permit a plasma arc to occur between the sections of the MOV thereby endangering the electronic device to which the surge arrester is attached.
Attempts have been made to solve the problems described above by individually fusing each of the MOV modules within a given surge arrester. For example, U.S. Pat. No. 5,808,850 to Carpenter discloses a three phase MOV surge arrester that employs a plurality of MOV modules, each of the MOV modules having an associated external fuse. Improper fuse coordination with the MOV characteristics or improper substitution in the field personnel can permit a plasma arc to occur between the sections of the MOV thereby endangering the electronic device to which the surge arrester is attached. In addition, fuses of this type are rated on a current, i.e. ampere basis and as a consequence do not operate or otherwise measure the thermal state of the MOV module. Thus, over time, and as the MOV module begins to deteriorate, an external fuse is inadequate to protect the MOV from failure as the MOV will overheat long before the fuse is triggered, if ever.
It is therefore, an object of the present invention to provide an improved MOV module for use with surge arresters that overcomes the disadvantages and shortcoming found in the prior art.
It is further object of the present invention to provide an improved MOV module that will not greatly overheat, rupture or otherwise fail when subject to extreme voltage or current conditions.
It is further object of the present invention to provide an improved MOV module that will permit the manufacturer to integrate, within the MOV housing, both thermal and overcurrent disconnect capabilities which will prevent overheating, rupture or otherwise catastrophic failure when subject to extreme voltage or current conditions.
Other objects and features of the present invention will be apparent from the following disclosure.